Technical Temporary Works Toolkit | Part 13 thestructuralengineer.org

Temporary Works Toolkit

The Temporary Works Toolkit is a series Part 13: The importance of articles aimed primarily at assisting the permanent works designer with temporary works issues. Buildability – sometimes of understanding referred to now as ‘construction method engineering’ – is not a new concept and one always recognised as vital to the realisation construction of one’s ideas; it ought to be at the forefront of an engineer’s mind.

methodology www.twforum.org.uk

Damian McBride BE, ME, MIStructE Director – Construction Engineering Manager (UK), Robert Bird Group

Introduction A structure in its permanent state cannot be ‘wished into place’. Appropriate consideration must be given to construction methodology so that permanent works can be optimised, temporary works minimised and trade interfaces simplifi ed. The result can be programme enhancement, increased site safety and, overall, a more economic build. Indeed, in the UK, regulations place a statutory duty on permanent works designers to consider the construction methodology1. Nonetheless, ‘It’s the contractor’s problem’ is Figure 1 Nova Victoria, London, under construction. Ground-fl oor slab has been partially constructed, allowing large a sentiment that is sometimes still apparent zones for excavation, and its initial extent has been designed for optimal control of ground movements along with provision of logistics. Cores can be seen being launched from B1 Level, allowing superstructure to proceed with regards to buildability. in parallel with basement excavation If appropriate considerations are not made at the right stage of design and procurement, the accuracy of the cost plan The following is a list of general, high-level will be of more general use. An understanding can be aff ected, as downstream changes to considerations that relate to construction of – and consideration for – construction permanent works, and unforeseen temporary methodology and optimisation of temporary/ methodology is important if informed works, can introduce additional expense. permanent works, with a number of examples decisions are to be made at an appropriate If the permanent works can be designed provided. It is by no means an exhaustive list. stage of design development. for the optimal construction methodology, Issues are not examined in great detail and more costs may be realised up front and any the examples do not provide any ‘rule-of- Construction methodology temporary works limited. This is benefi cial, thumb’ solutions. The considerations listed considerations as temporary works can be expensive and are intended to aid permanent works design Site location cumbersome and their coordination with engineers in their understanding of the type 1) Where is the site and how could access to permanent works can be complicated. of questions that need to be asked during the it aff ect construction logistics, such as the Transfer of loads between permanent and early stages of design. delivery of construction plant? For example, temporary works can also be complicated, The examples given are based largely on large piles will require large, heavy piling rigs. expensive and risky – in terms of structural projects constructed in London. However, it Will access constraints necessitate complex behaviour, ground movement and safety. is hoped that the type of thinking presented lifting operations and/or extensive temporary

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Figure 2 works and could these eff ects be reduced Extensive, temporary backpropping beneath through the design of more numerous, ground-fl oor slab, smaller piles? required to support heavy plant above

2) What buildings currently occupy the site and how will their demolition aff ect surrounding properties? If there are party or boundary walls reliant on existing buildings for stability, what temporary measures will be necessary to maintain this during and after demolition? How will design and construction of the permanent works need to work around these temporary measures? How will restraint be transferred into the permanent works and what infl uence will this have on permanent works design?

3) How sensitive are surrounding features to ground movement? How will this constrain Figure 3 Figure 4 Crane base If possible, connecting heavily loaded crane the demolition and construction methodology to be incorporated ties to permanent works columns (top) can be into raft of three- more effi cient solution than temporary bridging and how can permanent works design be level basement, columns (bottom) built top down. adapted accordingly? Initial, localised From initial inception of the chosen section of permanent raft holistic solution, permanent works design (built inside coff erdam) can for the Nova Victoria project in London be seen was optimised for a top-down construction methodology, and this was governed in part by limitation of ground movement suffi cient to protect adjacent buildings, along with critical buried services and infrastructure (Figure 1).

Site logistics 1) How will construction plant move around the site during construction, and how will this aff ect the permanent works in its complete or partially complete state? On a tight inner-city site with a deep basement, early localised construction of permanent works (e.g. ground-fl oor slab) may prove optimal to provide logistical space that would otherwise be either unavailable until later Craneage of a suspended steel crane grillage on plunge in the construction programme, or reliant 1) Where might tower cranes be located columns. Either solution (or any others) would on temporary works to facilitate temporary and how could this aff ect permanent works require appreciable coordination between gantries and the like. The initial extent of design? On a constrained site, tower cranes temporary and permanent works. Therefore, permanent works will be subject to a very can be a hindrance. Consequently, there the early establishment of an appropriate diff erent regime of loading and response can be a desire to place them on cores, or strategy was important. compared to the fi nal situation (Fig. 1). even climbing inside cores as they’re being constructed. Such systems can place heavy 3) Given the likely locations of cranes 2) What construction loads might the demands on core design (locally and/or and pick-up points in relation to zones of permanent works be subjected to? Figure 2 globally) and it is benefi cial to recognise this construction, what are realistic lifting limits? shows backpropping beneath a ground-fl oor early in the design process. Such considerations can infl uence the design system carrying a crawler crane installing of heavy steel or precast concrete elements very heavy steel elements above, where 2) How will tower cranes be founded? The by dictating joint locations. fl oor design had not considered such a load incorporation of a crane base into permanent case. Eff ective, retrospective backpropping works is obviously an effi cient solution. The 4) Tall cranes will require tying back to design is not always simple, as props must example shown in Figure 3 required the the superstructure as it is built, often be collectively stiff enough to prevent design and construction of a temporary necessitating complex temporary works in overloading of stiff er elements, and the result sheet pile coff er dam (including propping). order to carry the heavy loads to stability can be excessive prop numbers and/or However, after a detailed study that included elements. Could permanent works be utilised sophisticated (i.e. expensive) prop preloading consideration of programme and site logistics, for this purpose (Figure 4)? arrangements. this solution was chosen over the alternative

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Figure 5 5) How will tower cranes be installed and Infl uence of pile location, and clearances to existing removed? It is often necessary to employ a structures, on temporary large mobile crane, but where could this be works requirements placed and how could this aff ect permanent works design?

6) Further to consideration of lifting limits, is tower crane usage feasible for all permanent works elements? The example given in Fig. 2 is a situation where site tower cranes were insuffi cient for the given weight of steel members; hence, the requirement for crawler cranes traffi cking over the permanent works.

7) Will tower crane usage infl uence overall building height? Height may be aff ected by planning restrictions and the necessary protrusion of a tower crane may subsequentlyy limit the permanent works.

Piling 1) What type of piling system is likely? In London, for example, a relatively small number of large-diameter, deep piles will likely need bentonite due to their interface with Thanet sands at depth. This methodology will necessitate an appreciable site footprint taken up by a bentonite farm, with potentially adverse eff ects on site logistics (and, therefore, programme). As an alternative, the possibility of utilising a larger number of small-diameter piles founded in clay (and, therefore, not requiring bentonite) should be explored early in the design process such that informed decisions may be made.

2) How will pile position infl uence the amount of temporary works required? A number of recent central London developments involve an existing site with a single-level basement, with the demolition of existing buildings on the site and construction of new buildings with a deeper basement (typically up to three levels or more). Can existing basement walls be used for earth retention during construction, thus saving extensive temporary works? This question depends on the alignment of the new basement wall in relation to the existing, and relies on an understanding of piling methodology. As shown by Figure 5, it is also important to understand the infl uence of piling platform creation. Placing new walls inside existing (as opposed to demolishing the existing to create more space for the new) can have a limiting eff ect on new basement fl oor area. Again, early identifi cation of such considerations allows options to be weighed up and informed initial decisions made.

3) How will piling plant be supported? Piling rigs are heavy equipment, often

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accompanied by crawler cranes (for reinforcing appreciable architectural implications (e.g. cage and sleeve handling), spoil-removal accommodation of plunge columns in core

plant (excavators, lorries, etc.) and concrete Figure 6 walls) along with critical considerations of trucks. Loads applied by such plant can Suspended staged temporary stability (e.g. a 10-storey-high temporary be exaggerated by the dynamic eff ort of piling platform, core supported vertically by plunge columns designed to excavation, sleeve removal, and the like. limit piling rig with limited temporary buckling restraint, surcharge on It is not always possible for piling platforms adjacent brick- supported laterally by incomplete basement to be ground-bearing and, in some cases, even arch tunnel fl oors and subject to wind loads along with a if they could be, the surcharge applied can climbing crane system!) overload sensitive buried features. Suspended piling platforms (Figure 6) can be very complex 2) Is core slip-forming likely? For a tall building, and expensive; therefore, if permanent works particularly one with a concrete core and can be altered to simplify or eliminate such steel-framed fl oors, slip-forming of the core is platforms (e.g. by shifting pile locations, or by likely to be an option favoured by the concrete using a larger number of smaller piles), the frame contractor – due to programme and trade benefi ts of doing so should be weighed up interface benefi ts. This may mean that core early. wall construction appreciably precedes fl oor construction and, if so, core temporary stability 4) At what stage is piling expected to start? is reliant on core walls only, i.e.: What constraints will be in place then  globally, the stabilising weight of surrounding (e.g. limited headroom) and how will these fl oors is not in place to counteract any constraints infl uence the available type of piling overturning loads rig? How might this infl uence the optimal pile  locally, with no fl oors in place, wall panels are diameter and depth (including consideration required to span horizontally (between return of programme and cost) and could permanent walls) under lateral loads, and resist any works design be altered to achieve an optimal torsional eff ects. solution? analyses, and above-ground construction. There are many aspects of permanent works In both these examples, elements of wall Basement excavation/construction design potentially eff ected by top-down reinforcing design may be governed by 1) How have ground movement limitations been methodology. Questions to be asked include: temporary considerations and/or temporary determined? It is often the responsibility of  Are permanent plunge columns feasible? propping may be required. the permanent works engineer to determine They will only be so if there is suffi cient appropriate parameters for specialist design architectural space to accommodate them, 3) How will concrete frame construction of basement temporary works and, in fulfi lling and consideration needs to be made of the be staged? What eff ects will that have on this role, it is important to have an appreciation tolerance within which they can be installed temporary stability and could stresses become of cumulative eff ects from, say, demolition (i.e. plan position and verticality). ‘locked in’ due to constraint of behaviour such temporary works, piling, basement temporary  What is the likely staging of basement as shrinkage, creep and axial shortening? works, excavation, basement permanent works, construction? For example, in a three-level Figure 7 indicates a mid-rise, deep basement and the transfer of loads between phases. basement it may be programme-optimal to building constructed in two halves – this was fi rst cast the B1 level slab on grade, excavate due to full site possession being delayed by 2) What is the likely geometrical relationship down to B3, cast the lowermost slab at that demolition/vacant possession complications. between temporary basement props and level, before coming back up and completing Stability in the permanent situation is provided permanent works? It is benefi cial for the B2, then ground fl oors. An understanding of by a central core, with the frame being a fl at permanent works designer to be aware of the such sequencing is necessary to accurately slab. likely method of prop installation and removal analyse and design permanent elements Only half the core was constructed with the such that, if necessary, appropriate allowances such as basement walls, columns and fi rst stage, and its position within the incomplete can be made in the design of elements such as fl oors, allowing, for example, for all stages structure temporarily rendered the system piles, basement fl oor slabs and capping beams. (temporary and permanent) of axial loading highly torsional. Following a detailed fi nite- and buckling restraint. element (FE) analysis, in addition to redesign 3) At what stage will loads be transferred  What is the likely arrangement of moling of core reinforcing and the introduction of between temporary props and permanent holes? These are necessary to facilitate temporary propping, it was found necessary props (i.e. fl oor slabs), and what is the likely excavation beneath cast slabs, but to redesign connections between precast state of slab completion? At the time of load their presence introduces temporary columns and fl oor slabs in order to ensure transfer, fl oor slabs may be partially built and/ penetrations and, therefore, for the slabs, a suffi cient lateral stiff ness. or have temporary openings and, therefore, be distinct regime of loads and responses. When the second half of the building was subject to design actions that vary from the constructed, there was concern that, should permanent situation. Concrete frame construction the halves be tied together as construction 1) Is it possible for core construction to proceeded, constraint to axial shortening 4) Is top-down construction the optimal commence prior to basement completion? of the second stage could induce high local solution? This is not a simple question and it will This is one potential advantage of top-down stresses at the interface with the fi rst stage. only be answered by a thorough, holistic study basement construction, and was employed Again, a detailed FE analysis was required to of numerous factors, such as construction by the Nova Victoria project highlighted in determine such stresses to be manageable. logistics, programme, ground-movement Fig. 1. This method of construction can have

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Steel frames Figure 7 First half of Clarges building frame (London) completed Further to a number of the items discussed full height, with half-core’s position dictating torsional temporary stability system earlier, the design of permanent works for long-span steel-frame systems may often be governed by consideration of temporary eff ects. For the development of a holistic solution, it is important that the permanent works designer gains an early understanding of issues such as:  What size of prefabricated element can be delivered to site?  What is a feasible lifting arrangement and how will that limit the weight of prefabricated elements?  What stresses will prefabricated elements be subjected to during lifting operations and could any of these stresses be locked into the complete structure?  How will elements be connected at height and what temporary works will be required to facilitate this (e.g. temporary towers for long-span trusses, such as those shown in Figure 8)?  At each stage of erection, how will the stiff ness of temporary works (including their foundations, if applicable) contribute to the stiff ness of the global system? What will be the staged eff ect on the incomplete permanent works and how will this infl uence the complete structure?  What temporary systems will be required to ensure global stability at all stages of construction and how could these infl uence the staged behaviour of permanent elements?

Conclusion A number of examples have been provided of how infl uential construction methodology and temporary works can be on permanent works design. While it is seldom their responsibility to develop a detailed methodology, or to design temporary works, it is essential for the permanent works engineer to have suffi cient appreciation of such factors if they are to provide eff ective, holistic design leadership to a project, to maximise their contribution to project effi ciency, site safety and sustainability, and to fulfi l their statutory duties.

REFERENCE

E1) Construction (Design and Management) Regulations 2015, SI 2015/51 [Online] Available at: www.legislation.gov.uk/ uksi/2015/51/contents/made (Accessed: June 2017)

HAVE YOUR SAY

To comment on this article: Figure 8 Eemail Verulam at [email protected] Temporary towers for erection of long-span arch structure. Permanent works design was heavily infl uenced by Etweet @IStructE #TheStructuralEngineer temporary condition, with staged global analysis requiring detailed incorporation of temporary works constraints

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